737 VNAV profile with extended centreline

[Radar Love]

Something that's puzzled me for a while...

When you extend the runway centreline on approach, does anyone know what the VNAV vertical deviation scale on the ND (or EHSI for classic flyers) is actually telling you?

For example, if you're below profile according to the vertical scale, what is that scale referenced to? Is it where the track line intersects the magenta extended centreline (but how would this work if you're downwind and parallel to the magenta line?), or is it referenced to something else?

Thanks :ok:

[flyburg]

I would think it is probably referenced to the point you used to create the extended c/l with, but I'm curious for the answer as well!

[josh737]

Ur right... present position to the point u used to create the C/L.

[GW_04]

It will give you a profile from your position abeam the extended center line as if you were established on it. Because VNAV is always calculated from track miles off the magenta line it uses track miles to go on the line from your abeam position (as if you were on it so to speak).

An example: If you were turning base towards the extended line (90 degree intercept) and had around 5 track miles to run before intercept, it would show you as around 1500ft high if you were spot on profile. As you approach the line the pointer will count down and come nicely onto profile.

This works well if you turn outside the active way point.
If ATC vectors you inside the active way point then it will calculate your abeam position to the active way point and then back again.
I normally just bring the final fix to the top and intercept off that.

Another way of getting a good profile without the mental calc is to select LNAV when on an intercept, and it will give you the correct profile straight away as it now knows how many track miles you have to run.

Hope that helps a little. :ok:

[capt. solipsist]

RADAR LOVE,

The VNAV computes your vertical track on HDG SEL from the active waypoint (meaning the waypoint from where you created the extended centerline) to your PRESENT POSITION as if you're proceeding direct to it, not on any abeam position on the magenta line.

[flyburg]

Ha,

Now we have 2 conflicting answers.GW 04 and captain S. I have to say the answer of GW 04 made a lot of sense, also with past experiences. Could anyone elaborate further?

Thanks in advance

[capt. solipsist]

There's a saying that goes: In the land of the blind, the one-eyed is king :ugh:

[ImbracableCrunk]

It's a very good question you pose.

I do know that if you're on a base in HDSEL the VNAV will give you a completely different answer than if you have it in LNAV.

I've seen people "sneak a peek" by toggling LNAV when on a vector. That seems to jive with what gw_04 is saying.

Just so I'm on the same page: Assume you're on vectors 090HDG to RW36. Assume a "3-4-5 triangle" with a 3-mile base to intercept a 4-mile final to FI36. Your PPOS to FI36 from the base would be 5 miles.

If I understand gw_04, is saying VNAV would assume you have 4 miles to go and 900' high, if you're on profile. (Since it doesn't account for the 3-mile base.)

If what Capt S. is saying is correct, the VNAV would think you're going PPOS DIR to the FI36. You'd be 5 miles away on the base vector, so the VNAV would want you 1500' above FI36. To be on a 3-1 with a really sharp 90-degree turn to final, you'd actually have 7 miles (3-mile base + 4-mile final) to go, or 2100'. It would show you as 600' high (2100' - 1500').

Now I've totally confused myself. . . http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/badteeth.gif

Like I said, good question.

[BOAC]

Well, I never bothered to try and puzzle it out, 'cos I never used it.:) Being one of those old-fashioned pilots who knew roughly where he was and where he was going I would use that old magic mental 'DR' or the database generated map centre-line (dotted white) or approach final track (a beautiful Magenta) if I needed them and worked out my track miles and therefore required/actual energy levels as my vectors changed . A dying skill, I fear, but at least you don't have to agonise over what all the coloured lines mean, nor push too many buttons when you should be thinking. All the F/Os seemed to be hell bent on extending the c/line but I suspect they didn't really have a clue what it meant. With 2 CDU's and 2 HSI's I just used to let them get on with it. With 1, I would break their arm.:=

There is actually a neat way to get a continual 'miles to run' on the 737 by putting your 'aiming point' at the top of the legs but not executing. Most trainers will complain that you should never have the execute light left on, so a surreptitious selection of 'Progress' satisfies all but the most pernickity.

[Agaricus bisporus]

Now I've totally confused myself. . .

An all too common result amongst those who slavishly follow that misleading little profile thing without thinking past it, and there are an awful lot of them out there.

As BOAC correctly says the pilot should be keeping a permanent mental plot of where he is in all dimensions. Multiplying by three is hardly a challenge, so why do so few do it?

Still others take great pains to construct a complex imaginary but apparently comforting "circuit" that of course they don't actually fly because radar takes them somewhere else, and still act on the erroneous information the FMC innocently produces!

Whatever happened to airmanship?

[ImbracableCrunk]

Good question. I went from a world where we used the Mk. I iBall to shoot a visual approach to the lovely world of flying in Asia, where if there isn't a formula for how you can't make it needlessly complicated, you're not trying hard enough.

I miss calling the field in sight and clicking off the automation for a visual approach.

[flyburg]

Sigh...

Here comes somebody that wants to learn a little about the airplane and or systems ( also considered airmanship) and here comes the old fart, geeser or what not. Doesn't understand the question or doesn't know the answer and starts a rant about airmanship and breaking arms!!

Congratulations, great airmanship!!!!

[BOAC]

Don't fret, FB - like London buses, I expect three geeks will be along shortly to help.

"doesn't know the answer" was indeed correct (you could add "want to" as well), and I cannot understand the fixation with using something that gives you information for a profile you will not fly. My 'old fart's' solution is far better - and I understand it.

How about RTA?

[flyburg]

Actually,

I too use the technique you describe. However I use the C/L funtion as well once in a while to clean up a cluttered picture or for whatever reason. I too saw the little gismo give weird readings and wondered about it.

Somebody asked the question, the answer is actually interesting! Does that make me a bad "follow the magenta line" pilot?

[Agaricus bisporus]

No no flyburg, far from it. The magenta line you generate with an extended centreline is surely the most logical reaction to radar vectors once it is apparent that they will result in an approach as opposed to a return to the STAR.

No way are you a magenta line junkie doing that - as you say it declutters the screen, it is the pressing need for a magenta line to follow at all times and to replicate your anticipated but unpredictable circuit that feeds you duff info on what you aren't actually doing that we are querying. That is making unwarranted assumptions, which was always conveyed to me as bad airmanship.

I use extended centreline whenever I can to clear the screen of ancient data and when on vectors to an approach it is the best tool one has, but it requires a bit of interpretation which is perhaps where the magenta line/instrument junkies begin to get out of their comfort zones and when their crutches like that misleading profile display start to cause disruption.

There is no substitute for situational awareness based on first principles as opposed to relying arbitary magenta lines and computers. The old farts amongst us call it Airmanship.

[arba]

flyburg :Does that make me a bad "follow the magenta line" pilot?


dear flyburg,
this year is my 17th year flying magenta line .. no problem ! hahaha !

safe flight & regards,

[flyburg]

Hello Agaricus Bisporus,

All in good humor ;)

Young lads come here to ask questions, some people give serious answers.

However, occassionally enters (I'm not implying BOAC now as he usually has some interesting input) the old fart/geeser(more often than not on this board a threeholer). Feels a need to establish his superior aviation skills. Either knows the answer and enters into a rant about what a disgrace it is that young blokes aren't being properly taught anymore. He was, of course, thaught the proper way back when learning to fly the tiger moth or,

Doesn't know the answer and starts a rant about how people can't fly anymore without the gismo's and when he was thaught to fly on the old tiger moth they didn't have the gismo's and they did just fine, thank you!

Anyways, I too can fly without the gismo's, however, occasionally, I too wonder about what a gismo is displaying. I just find it interesting. Trying to find out as much as possible about the airplane is good airmanship too.

If someone however goes of the deep end in his questioning about unimportant things, this should also be adressed! Works better though when done in a non sarcastic way. Also good airmanship ;)

Well, still curious for the answer!!

[GW_04]

Gidday RL,

If I may make a further comment to the extended center line, next time you are on base (90 degree intercept) and outside the active way-point, allow yourself to get a little low and level off momentarily and you will find the VNAV pointer will become stationary which will demonstrate it is not calculating track miles to the next way-point (In HEADING SELECT that is). Give it a try.

Just to clarify further, VNAV calculates profile in distance to the active way-point in LNAV or on the magenta line that is, which is why it is stationary when on a 90 degree intercept in HEADING SELECT.

And by the way, there is nothing wrong with knowing the magenta lines capability's as Mr Boeing (and others) put it there for us to use as a great tool for situational awareness as long as we use it wisely. As for the mark one eye ball I like to put say a 10 mile ring in the fix page off the runway threshold and use that as a 3000' mark and eyeball the white map range scale for extra miles. What ever smokes your tires and works on the day I reckon.

PS. It would be great if some one could quote a reference to this question's answer some where.

Cheers :ok:

[capt. solipsist]

GW 04;

The reason the VTK pointer does not (seem to) move when you MOMENTARILY level off on base is because the track distance change from your present position to the next active waypoint is incremental.

[GW_04]

Capt,

Yes the track distance change viewed on the ND is incremental in decimal of 0.1, however VNAV calc is continuous.

What about when your making a VNAV approach/arrival of whatever type (NDB,VOR,LLZ,GNSS,RNP,LNAV/VNAV etc..). I can assure you VNAV is calculated continuously to the foot. It takes into account programmed decent winds (unless a direct too has been executed then it uses current wind), and from that uses the expected ground speed to work out the profile. It will not take into account unexpected tail winds or excess energy from speed reductions not put into the FMC.

Another example is how the FMC calculates turning radius when turning at a waypoint in LNAV. If the turning waypoint (eg. Initial Approach Waypoint) has on the LEGS page a speed of 180Kts, and you are doing 210Kts, you can bet it will shoot through the Localizer onto final.

It does not continuously update as many parameters as we would like it to do, however if you tell it whats going on in the bigger picture it does a very nice job all the same.

Cheers